Moving coil motor

ABSTRACT

An improved linear motor includes a motion transducer disposed within the core section of the magnetic circuit for the shuttle of the linear motor to provide a velocity output signal substantially unaffected by magnetic fields and eddy currents associated with current in the linear motor coil. The positional adjustment of the magnetic transducers carried by the shuttle of the linear motor is facilitated by the cam action of an adjustment tool which fits within a recess in the shuttle and engages a transverse slot within the support for the magnetic transducer.

SIG-'13 SR nited States Patent 1 1 Lloyd [11] 3,745,386 [451 July10,1973

[ MOVING COIL MOTOR [75] Inventor: William J. Lloyd, Belmont, Calif.

[73] Assignee: Hewlett-Packard Company, Palo Alto, Calif.

[22] Filed: Feb. 28, 1972 [21] Appl. No.: 229,890

[52] U.S. Cl. 310/13, 336/136 [51] Int. Cl. H02k 41/02 [58] Field ofSearch 336/136; 310/12-14, 15, 27; 179/1155, 119, 120

[56] References Cited UNITED STATES PATENTS 2,442,016 5/1948 Poole310/29 X 3,470,399 9/1969 Johnson et a] 310/13 3,054,976 9/1962 Lipshutz336/136 3,440,459 4/1969 Pitt et a]. 310/15 FOREIGN PATENTS 0RAPPLICATIONS 789,726 1/1958 Great Britain 310/27 789,725 1/1958 GreatBritain 310/27 Primary Examiner-D. F. Duggan Attorney-A. C. Smith [57]ABSTRACT An improved linear motor includes a motion transducer disposedwithin the core section of the magnetic circuit for the shuttle of thelinear motor to provide a velocity output signal substantiallyunaffected by magnetic fields and eddy currents associated with currentin the linear motor coil. The positional adjustment of the magnetictransducers carried by the shuttle of the linear motor is facilitated bythe cam action of an adjustment tool which fits within a recess in theshuttle and engages a transverse slot within the support for themagnetic transducer.

1 Claim, 5 Drawing Figures Panama m 1 0 1m sum 1 as 3 CONTROL CIRCUITMOVING con. MOTOR BACKGROUND AND SUMMARY OF THE INVENTION Certain knownlinear actuators for radially positioning the magnetic transducers of adisc memory include a velocity transducer centrally disposed within theshuttle and protruding beyond the ends of the core. Because the fringingmagnetic fields present around the ends of the core structure and theeddy currents within the core structure produced by the current in thelinear actuator coil are coupled to the transducers, the resultantvelocity signal produced thereby is nonlinear over the range of shuttlepositions. A pair of velocity transducers, each suffering the sameeffects of fringing magnetic fields and eddy currents, are commonly usedin side-by-side location and in opposing connection to cancel out thenonlinearities caused by the fringing fields. However, this increasesthe moving mass coupled to the shuttle and also reduces the quantity ofmagnetic material in the central core region about the velocitytransducers.

These disadvantages of the prior art techniques are overcome in thepresent invention which uses an improved velocity transducer that isshorter than the core section and that is adequately shielded from theeffects of fringing fields. In addition, the shuttle includes adjustmentmeans for altering the static position of the magnetic transducer on thedisc memory for a given translational position of the linear motor.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of the linearmotor of the present invention;

FIG. 2 is a sectional view of the velocity transducer of FIG. 1;

FIG. 3 is a side view of the shuttle of the motor of FIG. 1;

FIG. 4 is an end view of the shuttle of FIG. 3; and

FIG. 5 is a perspective view of the adjustment tool for the shuttle ofFIG. 3

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there isshown a magnetic structure which includes a pair of magnets 9, 11 spacedside by side between end plates 13, 15 with a cylindrical core 17attached to the rear end plate 13 and protruding through an aperture 18in the front plate 15. The radial spacing between the core 17 and theaperture 18 forms an air gap 19 in the magnetic circuit across which aradial field is produced. Shuttle 21 includes a pair of laterally spacedsupport members 23, 25 which support the shuttle on rails (not shown)for movement in a longitudinal direction. The shuttle 21 includes acoiled conductor 28 which is helically wound on a nonconductivethin-walled portion 31 of the shuttle 21 to form an electromagnet forinteraction with the radial field in the air gap 19.

Disposed within the axial bore 33 of the cylindrical core section 17 isa velocity transducer 35 which includes a pair of windings 37, 39disposed about a hollow stainless steel bobbin 41. A cylindrical magnet43 which serves as a source of magnetic field is mounted within thehollow bobbin 41 for translational movement therewithin in response tomovement of the shuttle 21. The magnet 43 is rigidly coupled to theshuttle 21 near a central location on the front plate 45 of the shuttleby a coupling rod 47. A magnetic shunt 49 of soft iron and having anaperture therethrough for the coupling rod 47 is disposed within theaxial bore 33 in the cylindrical core 17 adjacent the end of thewindings 37, 39. This shunt assures that stray magnetic fieldsassociated with the current in the coiled conductor 28 of the linearmotor are not coupled to the windings 37, 39. Also, the surroundingmagnetic material of the core 1'7 serves as the flux return path for themagnet 43.

As shown in FIG. 2, this transducer includes a bobbin 41 on which iswound a set of tapered windings 37, 39. The density of these windings(i.e. the number of turns of the winding per unit length of the bobbin)varies substantially linearly in opposite directions from opposite endsof the bobbin 41. The source 43 of magnetic flux may be attachedsecurely to coupling rod 47 using a suitable potting compound such asTeflon-loaded Delrin cast about both the magnet 43 and the coupling rod47. This enhances the slidable movement of the magnet source 43 withinthe bobbin 41 (formed, for example, of stainless steel) without the aidof lubricants and without producing wear particles. The thin-walledbobbin 41 of stainless steel conducts eddy currents due to the movementof magnet 43 but provides a superior bearing surface for the slidablemagnet 43.

It can be shown that the voltage generated by each of the coils 36, 39in response to movement of the magnet 43 is a function of the rate ofchange of position of the magnet 43, its flux and the number of turns ofthe coil 37, 39 intercepted by the changing flux. Thus, for a givencoil, the voltage is as follows:

where I) is the flux from magnet 43 (constant) and N is the number ofturns. For a tapered coil:

(2) where Velocity is dx/dt.

But, for a linearly-tapered winding:

dN,/d.x KJc Thus, the voltage for each of the coils is:

e I Kx V; and

e, I Kx V The coils 37, 39 are connected in series-opposingconfiguration and the output voltage thus is:

mu l 2) Since (x x is the length (Lm) of the magnet 43, the output is:

E 2K V4 Lm This analysis assumes that the length (Lm) of the magnet 43is short and disposed entirely within the length of the bobbin 41 forall movements of the magnet 43 in response to translational movement ofthe shuttle 21.

The coils 37, 39 may be wound simultaneously from opposite ends of thebobbin using a pair of wire guides which are moved along the length ofthe bobbin 41 from the opposite ends at the rate which is suitable forestablishing the linear taper of the winding density with length alongthe length of bobbin 41. It should be noted that the usable strokelength is L Lm. Since Lm is not a function of the coil length (L), theratio of stroke to the length of the bobbin 41 can approach unity. Thus,for a transducer of over-all length of approximately 4% inches and amagnet 43 of approximately l-inch length, the usable stroke length isapproximately 3 inches.

In operation of the linear motor of the present invention, a signal isapplied to the coiled conductor 28 through flexible connections (notshown) to establish an electromagnetic field about the conductor whichinteracts with the radial permanent magnetic field within the air gap 19to produce lineal movement of the coiled conductor 28 and of the shuttle21 that is attached thereto. The shuttle 21 carries with it thecylindrical magnet 43 within the bobbin of transducer 35 to produce inthe windings 37, 39 of the transducer an electrical signal which isindicative of the direction and velocity of the shuttle 21. Thesesignals, together with suitable digital means (not shown) for indicatingposition of the shuttle 21 along its lineal path of movement, are usedto control the movement of the shuttle in a conventional manner. Also,in order to maintain the inductance of the coiled conductor 28substantially constant over the linear travel of the shuttle 21, ashorted conductor 51 is disposed on the cylindrical surface of the coresection 17. This shorted conductor 51 is molecularly-deposited copper(plated or sputtered into place) which is overformed in a cylindricalrecess 53 and which is subsequently machined to provide a substantiallyuniform cylindrical core 17. The core 17 and end plates may then beplated with a suitable anticorrosion material such as nickel. Thus, whenthe shuttle 21 is located near its minimally extended position, theinductance of the coiled conductor 28 is decreased by the shortedconductor 51 which reduces the effect of the core 17 on the inductanceof the coiled conductor 28. Also, in order to insure accurate indicationof the velocity of the shuttle 21, the transducer 35 is disposed wellwithin the cylindrical core 17 in order to avoid the effects of straymagnetic fields and eddy currents due to current in the coil 28 whichtend to destroy the linearity of electrical indication of the velocityof the magnet 43.

Referring now to FIGS. 3 and 4, the shuttle 21 includes a pair ofvertically spaced support arms which are arranged to support a pair ofmagnetic transducers (not shown) with respect to the surfaces of a discmemory unit. Each of these lateral extensions 55, 57 includes a pair ofsubstantially horizontally oriented slots 59, 61, 63 and 65 whichcommunicate with one side face 67, 68 of the lateral extensions 55, 57.These slots are substantially parallel aligned with the axis of movementof the shuttle 21 and are arranged to hold therein a support member 69for a magnetic data signal transducer (not shown). In order to provideproper axial positioning of such magnetic transducers for a givenposition of the shuttle 21, it is necessary to adjust the axialpositions of the support members 69 within the corresponding slots59-65. For this purpose, a recess 71 is formed in each of the faces 67,68 of the lateral extensions adjacent the corresponding slots 59-65 toform a substantially semicircular recess that communicates with the slotin the face 67, 68. Each of the support members 69 includes a transverseslot 73 on an edge thereof which communicates with the face 67, 68 ofeach of the lateral extensions 55, 57. The recesses 71 and thetransverse slots 73 in each of the supports 69 thus furnish cammingsurfaces for movement of the support members 69 within the correspondingslots 59-65. An adjustment tool, as shown in FIG. 5, may thus beconveniently disposed within the recess 71 and transverse slot 73 toeffect translational adjustments of the support members 69 within theslots 59-65. This adjustment tool 75 includes a body portion having alever arm 77 and a protruding boss 79 which is arranged to seat withinthe recess 71. A protruding pin 81 located eccentric the rotationalcenter of the boss 79 within the recess 71 thus engages the transverseslot 73 within a support 69 such that rotational movement of the leverarm 77 produces translational movement of the support member 69 withinthe corresponding slot 59-65. Once the adjustment is properly made, aclamping plate 72 for each support member 69 may be tightened into placeto laterally squeeze the support member 69 firmly within the slot 59-65.

I claim:

1. Linear positioning apparatus comprising:

a magnetic structure including an outer magnetic circuit and an innermagnetic circuit, at least one of which includes a source of magneticflux, said inner magnetic circuit including a cylindrical core ofmagnetic material having an axial bore therein and being supported atone end thereof on said outer magnetic circuit, and said outer magneticcircuit including a' front plate having an aperture therein larger thanthe diameter of said cylindrical core for forming therewith an air gapfor supporting a radial magnetic field thereacross;

shuttle means mounted with respect to said magnetic structure forlineally slidable motion through the air gap, in a direction normal tothe radial magnetic field therein, said shuttle means having a conductorwound thereon for producing in response to applied electrical signal amagnetic field which interacts with the radial magnetic field in the airgap of said magnetic structure to produce linear motion of the shuttlemeans;

transducer means including a pair of windings disposed on a bobbinhaving an internal bore therein and being disposed within the axial boreof said core recessed therein from the ends thereof, said transducermeans including a source of magnetic flux slidably disposed within thebore of the bobbin and rigidly coupled to said shuttle means, thewindings of said transducer means being disposed on the bobbin withwinding densities that decrease substantially linearly with length alongthe bobbin from opposite ends thereof;

circuit means connected to the winding of the transducer means forapplying electrical signal to said conductor wound on said shuttle meansto control the linear motion thereof; and

a magnetic shunt disposed within the axial bore of said core adjacentthe bobbin therewithin and having an aperture for passage therethroughof the rigid coupling between the shuttle means and the source ofmagnetic flux for said transducer means.

t I! I III I

1. Linear positioning apparatus comprising: a magnetic structureincluding an outer magnetic circuit and an inner magnetic circuit, atleast one of which includes a source of magnetic flux, said innermagnetic circuit including a cylindrical core of magnetic materialhaving an axial bore therein and being supported at one end thereof onsaid outer magnetic circuit, and said outer magnetic circuit including afront plate having an aperture therein larger than the diameter of saidcylindrical core for forming therewith an air gap for supporting aradial magnetic field thereacross; shuttle means mounted with respect tosaid magnetic structure for lineally slidable motion through the airgap, in a direction normal to the radial magnetic field therein, saidshuttle means having a conductor wound thereon for producing in responseto applied electrical signal a magnetic field which interacts with theradial magnetic field in the air gap of said magnetic structure toproduce linear motion of the shuttle means; transducer means including apair of windings disposed on a bobbin having an internal bore thereinand being disposed within the axial bore of said core recessed thereinfrom the ends thereof, said transducer means including a source ofmagnetic flux slidably disposed within the bore of the bobbin andrigidly coupled to said shuttle means, the windings of said transducermeans being disposed on the bobbin with winding densities that decreasesubstantially linearly with length along the bobbin from opposite endsthereof; circuit means connected to the winding of the transducer meansfor applying electrical signal to said conductor wound on said shuttlemeans to control the linear motion thereof; and a magnetic shuntdisposed within the axial bore of said core adjacent the bobbintherewithin and having an aperture for passage therethrough of the rigidcoupling between the shuttle means and the source of magnetic flux forsaid transducer means.